Nuclear fission reactors use fuel pins which are loaded with pellets of fissionable nuclear fuel. The amount and concentration of fissionable or fissile material contained within the fuel pin is an important parameter for proper operation and maintenance of a nuclear reactor. Assurance of high quality and adherence to design specifications can advantageously be accomplished by inspecting fuel pins for uniformity and total content of the fissile material. It may also be desirable in certain cases to perform other types of inspections.
Fuel pin scanners are currently being used to inspect nuclear fuel pins to assure proper uniformity and amount of fissile material. Current technologies do not, however, provide the production speed or level of accuracy which is now required in producing fuel pins used in liquid metal fast breeder reactors. Plutonium recycle systems used in light water reactors also have similar expected need for high production capability and accuracy in inspecting fuel pins.
Fuel pin scanners are already in use in light water reactor fuel manufacturing plants. The early fuel pin scanners used passive systems which simply measured the natural radioactivity of the fuels. Such systems were very slow, thereby requiring large numbers of scanners just to inspect the output of a large light water reactor fuel plant.
The economical availability of californium-252 led to the development of nuclear fuel pin scanners which activate the fissile material using radiation. Such fuel pin scanners were capable of processing up to approximately 1,000 uranium oxide fuel pins per day. Such prior pin scanners used a single pass configuration which was relatively slow and provided limited accuracy.
The need for fabricating plutonium bearing nuclear fuels on a large scale arose with the liquid metal fast breeder reactor program. Scanning of plutonium bearing fuel pins used in such reactors has created special requirements which were not satisfactorily met by the prior art. Most significant of the problems was the need for greater accuracy in measuring the uniformity of fissionable material contained within the fuel pin. Liquid metal fast breeder reactor fuel pin scanners must not only detect rejectable defects but must also allow characterization of the fuel for identification purposes. Characterization of the fuel allows for the fuel to be more closely monitored during the manufacturing process. This in turn aids in the production of high quality and safe, fast reactor fuels.
The typical prior art light water reactor fuel pin scanner consisted of: (1) an irradiator containing one to five milligrams of californium-252; (2) mechanisms for transporting fuel pins sequentially through the irradiator and through one or two fission product gamma ray detectors; (3) sodium iodide, bismuth germanate or plastic scintillators; (4) a gamma ray transmission device for measuring gaps and nuclear fuel density; and (5) an on-line computer for collection and processing of data.
All prior art light water reactor fuel pin scanners measured fissile uniformity in a single pass of the fuel pin through the irradiator and detector. In this single pass configuration the fuel pins were passed near a irradiator containing a neutron source such as californium-252 which activates the fissile material to provide increased radioactive emissions therefrom. The activated fuel pin was then passed through a detector in a single pass. Such single pass systems were relatively slow because of the exposure time needed to sufficiently activate the fuel pin and the length over which activation occurred. Decreasing the exposure time to increase capacity required increasing the irradiation power which was not economical. Higher capacity could also be achieved through increased numbers of systems but this also was expensive and indicated the need for high capacity systems which addressed the problem in a new manner.
Such single pass activation and detection was also found impractical to achieve the increased accuracy necessary in producing fuel pins used in liquid metal fast breeder reactors. Liquid metal fast breeder reactors use nuclear fuel made with mixed oxides of plutonium and uranium, rather than the uranium dioxide fuels commonly used in light water reactors. Fuel pins made with mixed oxides of plutonium and uranium are more difficult to measure for fissile uniformity because the fissile loading of the fuel pellets is much greater and the thermal neutron activation commonly used with uranium dioxide fuel pins is not effective as an activating source of radiation.
Applying known light water reactor fuel pin scanner technology to fuel pins loaded with mixed oxides of plutonium and uranium requires using relatively large amounts (0.1 gram) of californium-252, as compared to 1-5 millgrams used in current light water reactor fuel pin scanners. This amount of californium-252, has a value in excess of $1,000,000 thereby rendering current fuel pin scanner technology uneconomical for fuel pins used in liquid metal fast breeder reactors.